Resolves error message shown.
The following error message is shown when worker_impl.py:199 is run:
```
WARNING:worker(RID,EXPERIMENT):py.warnings:/nix/store/77sw4p03cb7rdayx86agi4yqxh5wq46b-python3.7-artiq-5.7141.1b68906/lib/python3.7/site-packages/artiq/master/worker_impl.py:199: DeprecationWarning: The 'warn' function is deprecated, use 'warning' instead
logging.warn(message)
```
See test case – previously, the highest-priority pending run would
be used to calculate the timeout, rather than the earliest one.
This probably managed to go undetected for that long as any unrelated
changes to the pipeline (e.g. new submissions, or experiments pausing)
would also cause _get_run() to be re-evaluated.
From looking at the code, it wasn't obvious to me that this is
supposed to handle multiple calls to delete(). This is the case,
however, when for instance Scheduler.delete()ing a run, which
will then also be deleted again from AnalyzeStage.
This helps debugging the cause of TypeErrors arising from types
not handled by the HDF5 serializer, as the backtrace doesn't
otherwise include any useful information.
The docstrings are quite minimal still, but should already
help with navigating the different layers when getting
accustomed with the code base.
RIDCounter was moved to its own module, as it isn't really
related to the other classes (used from master only).
In larger experiments, it is quite natural for the same dataset
to be read from multiple unrelated components. The only situation
where multiple reads from an archived dataset are problematic is
when the valeu actually changes between reads. Hence, this commit
restricts the warning to the latter situation.
* just returning `None` as dummy device (like ExamineDeviceMgr)
is not explicit enough, certainly hard to debug
* introducing a special flag for the `build` action does not
seem the right place
